Semiconductor structure

ABSTRACT

A solid state image sensor has an array of pixels formed on an epitaxial layer on a substrate. Each pixel is relatively large so that it has a high light collecting ability, such as 40-60 μm, but the pixel photodiode is relatively small so that it has a low capacitance, such as 4-6 μm. Active elements of the pixel photodiode are formed in wells that are spaced away from the pixel photodiode so that the latter is surrounded by epitaxial material.

FIELD OF THE INVENTION

[0001] The present invention relates to electronics, and more particularly, to a solid-state image sensing structure.

BACKGROUND OF THE INVENTION

[0002] It is well known to use CMOS, active pixel image sensors in which incident light generates electrons that are captured by a photodiode in the pixel. When a high speed image sensor is desired, there is less time available for capturing light. One way to address this problem is to increase the illumination level, but this is frequently impracticable or undesirable.

[0003] Another approach is to use large pixels, since more photons impinge on a large pixel than a small pixel given the same field of view and field depth. However, in the prior art large pixels have a large photodiode and the capacitance of the photodiode is also increased. These photodiodes are usually operated in a voltage mode, and since V=Q/C, the capacitance rises as the voltage falls.

[0004] What is required is a large area pixel, but with a small sensing capacitance. U.S. Pat. No. 5,471,515 describes one approach to this requirement by putting a thin photogate layer over the light collecting part of the pixel. By applying a voltage to the photogate, the electrons are pushed through the transfer gate and into the sense node. However, there are practical disadvantages using this technique with large pixels. One is that a large photogate area is difficult to manufacture with high yields. Another is that pushing the electrons over a large area into the transfer gate (charge transfer efficiency) is also difficult to achieve. These problems may be addressed by modifying the manufacturing process, but this is not desirable since silicon fabrication costs rely on mass produced devices using a standard process.

SUMMARY OF THE INVENTION

[0005] The present invention is defined in claim 1. Other features and advantages of the invention will be apparent from the remaining claims and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:

[0007]FIG. 1 is a schematic cross-sectional view of one pixel of a prior art image sensor having a large area pixel and large area photodiode;

[0008]FIG. 2 is a similar view of a prior art sensor having a large area pixel and a small photodiode;

[0009]FIG. 3 is a similar view of a first embodiment of the invention; and

[0010]FIG. 4 shows a modified embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011]FIG. 1 shows the pixel layout of one known sensor. The pixel is large in that it has a width of typically 40-60 μm, as opposed to applications such as television which typically have a pixel dimension of 46 μm. The pixel is formed in a P-epitaxial layer 10 having a thickness of 4-5 μm. The P-epitaxial layer 10 is on a P substrate 12. The photodiode comprises an N-well 14, and is surrounded by a P-well 16 containing readout circuitry such as the NMOS transistor 18.

[0012] In the example of FIG. 1, the photodiode 14 is large in that it occupies most of the surface of the pixel. This leads to a high collection efficiency. Electrons e1-e7 are collected by the photodiode, while electron e8 goes to the P-well 16, which is connected to the supply, and is lost. However, the capacitance of the photodiode 14 is high.

[0013] In FIG. 2, a pixel of the same size and general structure has a photodiode N-well 14′ that is a small size, and thus of low capacitance. However, the collection efficiency is low. Electron el is collected by the photodiode 14, but all other electrons go to the P-well 16 and are lost.

[0014]FIG. 3 shows a basic form of the present invention. The circuit is formed, as before, with a P-epitaxial layer 10 on a P substrate 12, and with a pixel dimension typically 40-60 μm and a depth of 4-5 μm in the epitaxial layer 10.

[0015] The photodiode is provided by N-well 14′ that is a small size, and pixel circuitry is located within the P-well 16. However, the P-well 16 is spaced away from the N-well 14′, such that the N-well 14′ is surrounded by epitaxial material.

[0016] Due to the absence of P material in the vicinity, the majority of electrons, such as e1-e6 in FIG. 3, will diffuse in the epitaxial layer 10 and ultimately be collected by the N-well 14′. Electron e7 may find its way either to the N-well 14′ or to the P-well 16. Electron e8 will most likely find its way to the P-well 16 and be lost.

[0017] This effect occurs because the P-epitaxial layer is very lightly doped and is not connected to ground. Photogenerated electrons move at random by thermal diffusion until they are attracted by the positively charged N-well 14′ and are detected.

[0018] To maximize this effect, the epitaxial layer should be such that incident photons generate electrons within this layer. This process is wavelength dependent. Longer wavelengths penetrate deeper into the semiconductor. An epitaxial layer 4-5 μm thick is sufficient to collect light in the visible part of the spectrum. If infrared light is to be collected, the epitaxial layer should be made thicker, e.g., 10 μm.

[0019] For a pixel of the size range shown, a photodiode size of 3-10 μm is practical. The lower figure provides the higher sensitivity, but is constrained by manufacturing tolerances and also its ability to store photons. If too few photons are stored, the photon shot noise is increased and hence the ultimate signal-noise ratio of the sensor is degraded.

[0020] Thus, the arrangement of FIG. 3 combines a low photodiode capacitance with a high collection efficiency. The necessary change of structure in comparison with the prior art does not require any change in the manufacturing process, and thus permits low cost fabrication. It may require modification to the mask preparation stage, but this is only a one time cost.

[0021] An N-well is preferred for use as the photodiode collection node since it penetrates deeper into the epitaxial layer, and hence is more efficient in collecting electrons. However, in principle, the conductivity types could be inverted, and a P-well may be used in an N-epitaxial layer on an N substrate.

[0022] The use of a small photodiode with a large pixel size cannot be extended indefinitely. With larger areas, the electrons will recombine with hole defects in the silicon before being captured, and will be lost. The distance over which the electron will travel before recombination is known as the recombination length, and in modern silicon substrates is typically about 50 μm. Thus, a pixel size of about 60 μm is a practical upper limit with present silicon technology.

[0023]FIG. 4 shows a modified version of the foregoing embodiment. A thin layer 20 of P+ material is placed over the majority of the pixel. The layer 20 extends into the P-well 16, and hence is electrically connected to it. The P-well 16 is normally at ground potential, and so therefore is the layer 20. The layer 20 is at a lower implant depth and lower potential than the N-well collection node 14, and thus the electrons are more likely to go towards the N-well 14′ and be collected. For example, electron e7 in FIG. 4 is more likely than not to go to the N-well 14′, whereas electron e7 in FIG. 3 is quite likely to go to the P-well 16 and be lost.

[0024] The invention therefore provides an improved structure for image sensors combining large area pixels with low photodiode capacitance in a manner that is relatively straightforward to fabricate. 

That which is claimed is:
 1. A semiconductor structure for a solid state image sensor, comprising: a substrate of a first conductivity type; an epitaxial layer formed on one face of the substrate; an array of pixels formed in the epitaxial layer; each pixel having a potential well of a second conductivity type formed within the epitaxial layer to act as a photodiode collection node, and one or more pixel active elements comprising N or P wells within the epitaxial layer; and in which said active element wells are spaced away from the photodiode wells such that the photodiode well is surrounded on all its sides and at its rear by epitaxial material.
 2. The structure of claim 1, in which the first conductivity type is P and the second conductivity type is N; and the epitaxial layer is P-epitaxial.
 3. The structure of claim 1 or claim 2, in which the photodiode is small in relation to the area of the pixel.
 4. The structure of claim 3, in which the pixel has a width of 40-60 μm and the photodiode has a width of 3-10 μm.
 5. The structure of any preceding claim in which the epitaxial layer has a depth of 4-10 μm.
 6. The structure of claim 5, for use with visible wavelengths, in which the epitaxial layer has a depth of 4-5 μm.
 7. A structure according to any preceding claim, in which the surface of the epitaxial layer around the photodiode is covered, except for a narrow zone around the photodiode, by a grounded cover layer substantially thinner than said wells.
 8. A solid state image sensor comprising a structure in accordance with any preceding claim and image processing circuitry formed on a single chip. 